feat(audit): Parquet cold archive of expired AuditLog partitions before the purge (ScadaBridge#42)

Audit roadmap T2 (alog.md §15.2): with AuditLog:Archive:Enabled (default off)
the purge actor writes each expired monthly partition to Parquet under an
absolute AuditLog:Archive:Directory before switching it out, so audit rows
outlive RetentionDays.

- Parquet.Net 6.1.0 (fully managed; NuGetAudit clean) in central package
  management.
- AuditArchiveParquet: data file = the ten stored columns, OccurredAtUtc as a
  UTC microsecond timestamp plus exact OccurredAtUtcTicks, and ChainSeq/RowHash;
  tombstone file = the month's per-channel purge tombstones. Buffered row
  groups; the files are plain Parquet (DuckDB/pyarrow read them).
- AuditLogPartitionArchive (IAuditLogArchive): reads the partition's rows,
  chain heads and tombstones in one transaction under the chain lock SHARED,
  writes {Directory}/{YYYY}/auditlog-{YYYY-MM}-{stamp}Z.parquet,
  .tombstones.parquet and a .manifest.json (counts, SHA-256 per file, each
  month's head and prior anchor) via tmp + rename, manifest last. Archives are
  never overwritten.
- Archive before purge: SwitchOutPartitionAsync gains an internal guarded
  overload. Under the EXCLUSIVE chain lock it re-counts the partition and
  re-reads each month's LastSeq and THROWs 51002/51003 (nothing purged) unless
  both still equal the archive's, so the switch's checkpoint anchor is exactly
  the archive's chain end. The manifest is then marked Purged.
- Fail closed: a partition that cannot be archived is not purged; the failure
  takes the existing AuditLogPurgeFailedEvent / PurgeFailures path. Success
  publishes AuditLogArchivedEvent.
- Verify: file hashes and counts against the manifest, the chain recomputed
  from the archived rows and tombstones (AuditChainVerifier), and the chain end
  against the database anchor (Matches / Superseded / NotPurged / Mismatch).
- AuditLogArchiveOptionsValidator: Directory required when enabled and must be
  absolute; RowGroupSize, timeouts bounded.

No migration: the purger role already reads AuditLog, the head and tombstones.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

Directory.Packages.props

@@ -78,6 +78,9 @@
     <PackageVersion Include="NSubstitute" Version="5.3.0" />
     <PackageVersion Include="OPCFoundation.NetStandard.Opc.Ua.Client" Version="1.5.378.106" />
     <PackageVersion Include="OpenTelemetry.Api" Version="1.15.3" />
+    <!-- Parquet audit archival + the parquet format of `audit export` (ScadaBridge#42). 6.1.0 is fully
+         managed (Snappier / ZstdSharp / K4os LZ4, no native IronCompress) and NuGetAudit-clean. -->
+    <PackageVersion Include="Parquet.Net" Version="6.1.0" />
     <PackageVersion Include="Serilog" Version="4.3.1" />
     <PackageVersion Include="Serilog.AspNetCore" Version="10.0.0" />
     <PackageVersion Include="Serilog.Sinks.Console" Version="6.1.1" />